This sounds like a nightmare, MaysMum!
Obviously I know nothing about the details of what's happened.
The OSA wouldn't normally be involved - I'm assuming there was an issue about the admission arrangements.
When a school has converted to an Academy, who has the authority, i.e. next in line, to overturn an appeal?
Normally this would be the EFA/YPLA following a complaint and an investigation by them
I'm afraid I can't say whether the YPLA exceeded its powers.
I still think it might be worth contacting the DfE - not necessarily to make a complaint, but to see if they can advise. You could do this yourself, or you could ask your MP to intervene on your behalf.
You could also see if ACE have a view about how to resolve this situation. The Advisory Centre for Education (ACE) is a national charity that provides independent advice. They have a free helpline:http://www.ace-ed.org.uk/contact-us
(but getting through to them at this time of year can be difficult!).